In many cases the size of the particles being calcined or heat treated are less than 100 microns and in some of these cases the particle size is less than 10 microns.
In a typical application, variously termed calcining, heat treating or heat conditioning, the process may involve driving off volatiles, reacting or cross-linking the molecules of the solids or causing a change in molecular or crystalline structure due to rearrangement or phase change.
In most of these processes, it is important to transfer heat into the finely divided solids at a uniform rate. Moreover, in many of these situations, it is also necessary to remove a volatile that has been driven out of the particles.
For the processing referred to, various types of commercial furnaces have been employed, e.g. rotary kiln furnace, rotary hearth furnace, fluidized bed furnace and others.
The fluidized bed furnace is of particular interest for heating finely divided solids for the reason that, among its major characteristics, it provides very uniform heating and a relatively high rate of heat transfer into the particles. Generally, a fluidized bed furnace passes a gas upwardly through a gas-distributor plate containing ports to diffuse gas coming from a plenum chamber located below into and through the finely divided solids at flow rates sufficiently high to separate the solids from each other and to impart mobility to the solids constituting the bed. When this occurs, the bed becomes fluid-like in appearance and behaviour. The resulting milieu, termed gas-fluidized solids, is characterized by high convective rates of heat transfer from particle to particle as well as with respect to a heated or cooled surface with which the particles may be in contact.
The fluidized bed furnace has met with some commercial success in the heating of finely divided solids to effect a change in physical or chemical properties. In many of these applications, the temperatures involved are comparatively high, typically in the range of from about 200.degree. F. to about 2000.degree. F.
One major disadvantage which has limited the use of fluidized bed furnaces for these applications, particularly for products having a particle size distribution which includes a significant percentage of particles less than 100 microns, is that the fluidizing gas discharging from the bed, inevitably entrains a significant quantity of particles. This requires that the discharging gas be passed through one or more pieces of equipment, such as a baghouse filter, a wet scrubber, an electrostatic precipitator or equivalent, for particle removal. For operating temperatures above 350.degree. F., some of these approaches relying upon gas clean-up, are either not applicable or are extremely expensive.